The promo code should be entered for you on the Air Canada website, and you can try searching for the same dates you found on Google Flights. The price should now be cheaper.

Air Canada Terms & Conditions:

Book by February 28, 2017 at 11:59 pm from your city of departure in CanadaValid on Tango base fares.Travel from March 21, 2017 to June 28, 2017Maximum 9 passengers per bookingValid on flights operated by Air Canada, Air Canada Rouge and Air Canada Express.Codeshare flights are not eligible.Blackout dates apply:March 24, 2017 – March 26, 2017April 13,2017 – April 14, 2017April 16, 2017 – April 17, 2017May 18, 2017 – May 19, 2017May 22, 2017 – May 23, 2017